1. Field of the Invention
The present invention relates to a DC current detector which is simple and hence economical in construction but capable of detecting DC current with a high degree of accuracy.
2. Description of the Prior Art
A known circuit arrangement for detecting a DC current is such, for example, as shown in FIG. 1. In FIG. 1, reference character OPA indicates an operational amplifier; R and R.sub.1 to R.sub.4 designate resistors; and V.sub.CC and V.sub.SS identify voltages. A current I flows across the current detecting resistor R, and a potential difference thereacross is derived as a current detected voltage e.sub.0 from a differential amplifier circuit made up of the resistors R.sub.1 to R.sub.4 and the operational amplifier OPA. The current detected voltage e.sub.0 is given as follows: ##EQU1## where e.sub.1 and e.sub.2 are detected potentials at both ends of the resistor R. Since e.sub.2 =e.sub.1 +I.multidot.R, the current detected voltage e.sub.0 becomes as follows: ##EQU2## If the resistance values of the resistors R.sub.1 to R.sub.4 are selected to bear such relationships that R.sub.1 =R.sub.3 and R.sub.2 =R.sub.4, then ##EQU3## In Eq. (2), when R.sub.4 /R.sub.3 .noteq.R.sub.2 /R.sub.1, the first term on the right side represents an error and the coefficient of the second term on the right side, ##EQU4## causes an amplification error. To avoid this, it is necessary to employ high precision resistors to ensure that R.sub.1 =R.sub.3 and R.sub.2 =R.sub.4. Further, the potential e.sub.1 also has a marked influence on the detection error.
Moreover, this prior art DC current detector has the defect that its operation range is narrow. That is, this circuit calls for such relationships that ##EQU5## (i.e. in-phase input voltage to the operational amplifier)&lt;V.sub.CC. Usually a difference (V.sub.CC -V.sub.SS) between the voltages V.sub.CC and V.sub.SS applied to the operational amplifier OPA is substantially in the range of 30 to 40 V. In the case where the operating voltage is approximately in the range of 30 to 40 V setting the voltage V.sub.SS to 0 V. and ##EQU6## the detected voltage e.sub.0 diminishes, in consequence of which a post-stage amplifier is required. In this case, the detection error is also amplified.